The overall objectives of this project are to define the mechanisms of several coagulation reactions and to extract the kinetic parameters governing these reactions. Specifically, we wish to describe in detail how blood coagulation is initiated via the tissue factor pathway. This involves a determination of whether the zymogen form of factor VII has proteolytic activity. To do this, a new assay must be developed with which the activity of factor VII plus tissue factor can be assessed without generating activated factor X, a proteolytic enzyme known to catalyze limited proteolytis of factor VII. Similarly, the kinetics of the activation of factor X by activated by factor IX and factor VIII will be investigated. This reaction and the tissue factor accelerated reaction will be analyzed to determine the site at which these accelerators of coagulation exert their effects. We propose to do these experiments using chemically modified factor X radiolabelled in the carbohydrate chains with tritium. The reaction is monitored by following the release of acid-soluble radioactivity. A similar set of experiments will be undertaken to investigate the activation of factor IX by activated factor XI. Again, methods will be developed to evaluate this reaction without resorting to coagulation assays. When the kinetic parameters of these reactions have been extracted, a mathematical model will be constructed which may allow critical predictions to be made about the coagulation cascade.